3 EXPERIMENT Analysis of a freely falling body

EXPERIMENT 3
Analysis of a freely falling body
Dependence of speed and
position on time
Objectives
• to verify how the distance of a freely-falling body varies with time
• to investigate whether the velocity of a freely-falling body increases linearly with
time
• to calculate a value for g, the acceleration due to gravity
Apparatus
A Behr Free-Fall Apparatus and Spark Timing System will be used in this experiment. A
schematic representation of the apparatus is shown below in Figure 1 and a digital
photograph of the apparatus is shown in Figure 2 on the following page.
Electromagnet
Switch
Brass Cylinder
HV Wire
Spark
Switch
Paper
Spark Tape
HV
Pulser
Figure 1: Schematic of the Behr Free-Fall Apparatus
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Figure 2: The Behr Free-Fall Apparatus
Theory
In this experiment a brass cylinder is dropped and a record of its free fall is made. Before the
measurement, the cylinder is suspended at the top of the stand with the help of an
electromagnet. When the electromagnet is turned off, the cylinder is released and starts to
fall. Simultaneously, the spark timer starts to send evenly-spaced, high-voltage pulses
through two wires which are stretched along the cylinder’s path.
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At the time of each pulse a spark goes through the wires and the cylinder, leaving a mark on
the special paper tape that lies between the cylinder and one of the wires. The time interval
between two adjacent sparks, τ, is 1/60 of a second.
Measuring the distances between any two marks, Δy, and knowing the times between the
corresponding sparks, Δt, it is possible to calculate the average velocity during this interval
using the formula
Δy
v=
(1)
Δt
If Δt is small enough, we can assume that the velocity at any instant within this interval is
approximately equal to this average velocity. In the case where acceleration is constant, the
instantaneous velocity at the middle of the time interval Δt is exactly equally to the average
velocity of the object during the time interval Δt.
In general, for the motion of a body with a constant acceleration a, the velocity v is given by
the equation
v = at + v0
(2)
where vo is the velocity of the cylinder at t = 0. Since in our case the brass cylinder is falling
freely,
(3)
a = -g,
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where g=9.81 m/s is the magnitude of the acceleration due to gravity. The negative sign in
front of g is to indicate that the direction of the acceleration is in the negative direction (i.e.
downward). Therefore it follows from (2) that for a freely-falling body
v = v0 − gt
(4)
Thus g can be determined from a plot of v vs. t since the slope of any velocity versus time
graph is just the acceleration. The obtained value of g can then be compared with the known
value of the acceleration due to gravity. The position of the cylinder, y, as a function of time
is given by the standard equation for an object that is undergoing constant acceleration. If at
time = 0 the object has height yo and velocity in the vertical direction vo, then this equation
looks like
y = y 0 + v0 t −
1 2
gt
2
(5)
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Procedure
Using the free fall apparatus, drop the brass cylinder and record on spark tape the location of
the falling cylinder at a series of equally spaced time intervals, τ = 1/60 s.
The quantities needed to analyze the motion are the position (y), velocity (v) and time (t) of
the points on your spark tape. The choices of t = 0 and y = 0 are arbitrary and do not
necessarily refer to your first or last data points. However, as your object falls the position y
must decrease (becoming negative if necessary).
After you have performed the experiment, tape the paper strip to your lab table. Label the
points on your tape, starting with the third point from BOTTOM OF THE OBJECT’S FALL
as #25 and label them in DESCENDING numerical order. The reason for using the third
from the last point is you want to be sure you do not include the points obtained when the
brass cylinder is comes to rest at the bottom or when its begins to enter the putty at the
bottom of the apparatus. NOTE: Point #1 will not necessarily correspond to your very first
point, but the very first points are somewhat ambiguous anyway. It does not matter if you do
not use some of the first points. Next, put a ruler on your tape such that height y=0.0 cm
corresponds to point #25 and that height INCREASES as you move towards point #1.
Measure the locations of each marker with this ruler and write the y position on the tape next
to each marked point. Make sure you are labeling each point with its TOTAL distance from
0.0cm, NOT just with its distance from the previous point.
After completing this, transfer the y positions into the spreadsheet. Using the spreadsheet,
calculate the velocity at all points during the fall except for the first and last points. Be sure
you do at least one sample calculation to include in your lab report. As discussed in the
theory section, the instantaneous velocity at time ti are found by finding the average velocity
Δy
during the time interval of ti-1 to ti+1 for each of your points, v =
. In this equation Δy is
Δt
the difference between the position of the spark FOLLOWING and the position of the spark
PRECEEDING the spark for which you are trying to calculate a velocity. Similarly, Δt is the
time interval between the FOLLOWING spark and the PRECEEDING spark. For example,
y − y1
the instantaneous velocity at y = y 2 is v 2 = 3
.
t 3 − t1
Transfer the columns of “Time,” “y,” and “v” from your spreadsheet to Kaleidagraph for
graphing.
Prepare the following graphs:
I -- a graph showing Vy vs. t fitted with error bars and fitted with a best-fit line. Use the
procedure from Experiment 1. Make sure the equation of the best-fit line is on your graph.
From the slope of the line determine the gravitational acceleration, g, in cm/s2 and its
uncertainty.
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II -- a graph of y vs. time
After your perfected graphs are printed out, write a 2 to 3 sentence “Graph Analysis” at the
bottom of each graph. This should describe what is happening on the graph (“as the quantity
along the horizontal axis does such and such, the quantity along the vertical does this”), and
how what is happening on the graph relates to what actually occurs with those quantities
physically.
Questions
1. What is the y-intercept determined from your Graph I (or from the equation of its
best-fit line)? What does it mean?
2. From Graph I, or the equation of its best-fit line, find the time at which v=0 cm/s.
3. What is your value of the gravitational acceleration in cm/s2 determined from the
slope from graph I? Is this value compatible with the accepted value of the
gravitational acceleration? If not suggest a possible source of error (do not suggest
human error or a mistake).
4. What general equation describes Graph II?
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5. When the initial velocity is zero, what would you plot to make graph II linear: y2 vs.
t, y2 vs. t2, or y vs. t2? As always, justify your response.
Checklist
Your lab report should include the following eight items:
1)
your spreadsheet
2)
sample calculations
3)
plot of the height vs. time Graph II
4)
plot of velocity vs. time with slope (in cm/s2) Graph I
5)
interpretation of the two plots
6)
answers to questions
7)
one member of each group should turn in your spark tape record of the free-fall
8)
if your lab instructor tells you that you will need to use this data for next the next lab,
make sure you save your spreadsheet
Formulae, Definitions, and Errors for the Free Fall Experiment
DEFINITIONS
In this experiment you measure the position of a falling mass, m, at fixed time intervals. The
fixed time interval is determined by a high-voltage spark source. Read off the time between
sparks (τ) from the setting on the spark source.
You will measure the positions at each spark as y1, y2, y3, y4, etc. in centimeters [cm].
These positions will be referred to as yi.
To measure the speed at point a particular point “i” first calculate
Δyi = yi +1 − y i−1 .
For example for the sixth point
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Δy6 = y7 − y5 .
On your data sheet this is labeled as Δy(i). You are now ready to calculate the speed at point
i by dividing the distance Δyi by the time elapsed between the two points, or 2τ. So the
speed V yi is given by
V yi =
Δyi Δyi
=
[cm/s].
Δt
2τ
ERRORS
For each measured yi you assign an error based on how accurately you can measure that
point. This error is called δ y . This error determines all other errors in this lab. For this lab
and for the following formulae it is assumed that the error in τ and m are zero.
The error in Δ y at each point i is the same and is given by δ (Δy) = 2 δy
The error in the speed at each point i is
δ (V y ) = V y
δ (Δy )
Δy
= Vy
2δy Δy 2δy δy
=
=
Δy 2τ Δy
τ
.
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